The Zeiss 35mm f/2 ZM Biogon T*. This is small but not insubstantial lens for Leica M mount. I use this on my Sony A7R as well as my Leica M-P. And I love it. There is a perennial debate about this lens vs the Leica 35mm summicron. I’m not going to go into a technical debate here. Ken Rockwell and Steve Huff have written about this. Personally, for my needs, the Zeiss is all I want. I also have the Leica 35 1.4 Summilux, which is just sensational but the Zeiss comes really close for a tiny fraction of the cost. Very often, when I just want to carry one lens, I find myself reaching for the Zeiss for its light weight and sensational 3D pop, color and contrast. The Leica summilux scores on build quality, 1 stop advantage and the smoothness of out of focus transitions.